If you are lighting up all that charcoal at once, that would certainly turn your meat into charcoal. As mentioned, use the Minion method, so that at any given time, only a small amount of your fuel is burning, and the fire works it's way through the fuel like a slow burning fuse.

I also have to suspect, as others do, that your thermometer is not telling you the real temperature in your cooking chamber -- not even close. That much burning fuel and airflow has got to be making heat well above 300 degrees.